Tuesday, March 15, 2011

While my makeshift bench carries the burden of the Tektronix scope (effectively eliminating access for other things), I'll crack the case on another presumably operational unit for giggles.

Here's a Pelco VS512B twelve input electronic switch for multiple camera security monitoring systems. Operation is more or less identical to the simplest of analog sequencers, though instead of each stage passing a DC voltage corresponding to a potentiometer setting, each stage passes signal. Bandwidth is presumably up into the MHz, since it must carry video signal, low frequency has yet to be tested since the unit as stock is loaded with BNC connectors. Hole placement on this looks to be too congested for 1/4" jacks, so I may have to settle for RCA connectors on this thing.

Overall there seems a pretty light parts count here, considering. The twelve switching input sections at upper left, room for eight more. I believe far left is a gain recovery/buffer stage for the output. Power supply at right. Halfway into the frame is the rate pot, function switches below.

Input at left. Those diodes may play hell with signal integrity in the audio range, I'll leave them be for now. Red stripe actives are MCR102 Thyristors, presumably in a bistable daisy chain which translates to sequential switching. The locally (single channel) charged Thyristor could then tickle the PN3638A into conduction, sluicing the local input into the gain recovery stage for output.

Just speculation: The "ring" of Thyristors are the oscillator, cycle rate is dictated by the voltage supply charging up caps. Lock stepping this to an external sync pulse may be troublesome, but there is only one way to find out. Onto the pile with it.


the road dogg said...

Looks like the guts of decent sequencer/router all right. The PN3638 is general purpose so it should pass audio unless an emitter bypass cap was chosen to limit low-frequency gain (I'm speculating, can't really see what's happening just from those pics).

Even if the thing is band-passed for the Mhzs region only a cap or two per channel would need replacing.

crochambeau said...

I think I can scare up some BNC to RCA cables to test sound quality.

It also occurred to me that those diodes near the input leads may just connect to the MCR102s and not mingle with signal at all. I plan on spending some time with it this weekend.

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